Method for starting steam power plants



Unite States atent Inventor Lars Axel Andreas Chambert Kallhall, Sweden Appl. No. 757,608 Filed Sept. 5, 1968 Patented Oct. 6, 1970 Assignee AB Svenska Maskinverken Kallhall, Sweden a Swedish joint stock company Priority Sept. 11, 1967 Sweden No. 12496/67 METHOD FOR STARTING STEAM POWER PLANTS 2 Claims, 5 Drawing Figs.

US. Cl 122/406, 60/104 Int. Cl F22d 7/00 Field of Search l22/406(SU);

[56] References Cited UNITED STATES PATENTS 3,169,374 2/1965 Powell et al 60/l04X 3,187,727 6/1965 Rauese l22/406(SU) 3,262,431 7/1966 Hanzaler l22/406(SU) 3,368,533 2/1968 Kniza l22/406(SU) 3,4l L484 ll/l968 Kraus l22/406(SU) Primary Examiner-Carroll B. Dority Attorney-Sommers and Young ABSTRACT: A method of starting up large steam power plants after a temporary interruption. The method decreases the required startup time by utilizing the heat energy stored in the thicker walled steam piping and headers as much as possible while at the same time minimizing thermal shocks to either superheaters or headers, The method consists of suppressing normal steam flow through each superheater and respective header until the superheater has reached a temperature corresponding to its header. By this stepwise approach to normal operation the thermal shock to the parts are minimized and the total startup time is minimized by bringing each superheater into use as soon as possible, thus allowing an increase in heat flow rate.

Patented Oct. 6, 1970 3,532,079

Sheet 1 0195 FIG? Patented Oct. 6, 1970 Sheet I'll! II -l ccq oom

Patented Oct. 6, 1970 3,532,079

Sheet 4 of 5 I I I l l l I I l I l Sheet 5 of 5 Patented Oct. 6, 1970 METHOD FOR STARTING STEAM POWER PLANTS The startup operation of thermal power plants creates high thermal stresses in the heavy piping, particularly the superheater outlet headers, the main steam line and the turbine chest. While these stresses normally can be tolerated with infrequent unit startups, metal fatigue becomes a severe problem when the unit is to be operated on a two-shift basis with startup operations in the order of 100400 each year- --depending on unit size and how the pressure vessels in the unit are designed.

The object of this invention is to obtain a rapid startup of the steam generator and the turbine while minimizing the thermal stresses of the heavy metal parts.

The invention also gives a possibility to minimize the startup time and still protect the unit in a proper way independent of the actual temperature levels in the unit when it is started. The type of operation is question is so maintained that when the unit is taken out of operation for one shift, or periods which can be longer or shorter, it is bottled up in order to keep pressure and temperature as high as possible The heavily insulated, thick metal parts, such as the superheater headers and the turbine chest, will generally be at a very high temperature for a long time.

Calculations have, however, indicated that also big units in the order 300-500 mw. can be brought up to full load in the order of 40 minutes or perhaps less from the initial boiler firing during two shift operation.

FIG. 1 illustrates a typical steam generator. FIG. 2 shows typical gas temperatures (dashed), material temperatures of the coils (fully drawn) and material temperatures of headers (dash-dotted), and how they vary with time in a conventional startup after a limited stop. FIG. 3 shows a startup diagram according to the new system with time variation for pressure, flow and temperature. FIG. 4 is a schematic drawing which il lustrates the significant features of the invention. FIG. 5 illustrates how the factors of FIG. 2 vary when principles of the present invention are utilized.

In FIG. 1 the feed water is heated in the economiser l and supplied to the steam drum 3 through the feed water piping 2. Through the downcomers 4 and circulating pump 5 which may be omitted in a once-through or self-circulating boiler, the water is fed to the water drum 6 where it is distributed to the furnace walls 7. In the furnace walls the water is vaporized and through the top header 8 and piping the steam-water mixture (or steam) is returned to the steam drum 3. The water and steam is separated in the drum 3 and the steam is fed to the superheater sections. These consist of a low temperature section SH], 9 an intermediate section 8H2, l2 and the final superheater 8H3, 16. Each section is composed of an inlet header 11 and 15, tube banks 12 and 16 and an outlet header 13 and 17. Between the sections are desuperheaters and 14 located. The superheated steam is then led to the high pressure turbine with its throttle valve 19 and back to the boiler through the steam piping 18 and 21. The steam is heated in the first reheater section RHl, 23 and second reheater section RI-I2, 24 to full temperature. The outer low pressure parts consist of steam piping 26, turbine 28 and condensor 29. The steam may also be bypassed over the low pressure turbine 28 directly to the condensor 29 through a bypass valve 30.

The invention can best be understood by first discussing a startup without use of the system. When startup is about to begin, the headers and piping are hot, but the superheater and reheater pendants are, if the unit has been out of operation long enough, at approximately saturation temperature. The unit is fired at a rate which increases the saturation temperature 3C. per minute or another rate of this order which is not giving too high thermal stresses in the saturated system. At this firing rate the gas temperature entering the superheater may rise to but not exceed a level around 540C. There is essentially no steam flow through the superheater and reheater sections. The metal of the superheater and reheater pendants is heated by the gas flow. After an infinite time all the pendants would be at the 540C. temperature level as illustrated in FIG.

2 where the temperature rise in the first three tube banks is shown as a function of time. Since we are dealing with a transient, however, the leading pendant section 5H2 is heated most rapidly at first and, therefore, reaches a high temperature level much earlier than the rear pendant sections. The metal temperature of these leading sections, therefore, not

only precludes an increase in firing rate but depending on the.

mitted through the steam generator when the first superheater section after the furnace SH2 is heated up and the trailing sections are still not heated up. In this case the steam flow through the first section 5H2 is heated to a temperature close to the metal temperature in the pendant elements heated by the flue gases. The steam then flows through the headers after that section and may cause a very rapid heating of these.

At the same time the steam when passing through the trailing sections will cool down enough to cause a rapid cooling of the outlet header and the steam piping. This is not wanted especially as these sections when the turbine is loaded will be brought up to full temperature very rapidly as the loadincreases. Further will the turbine require high temperature steam also during the initial period of rolling and loading in order to be able to accept a rapid load increase without too high thermal stresses.

The basic concept of this invention, therefore, involves firing the unit at a rate to achieve an allowed and safe saturation temperature increase of for instance 3C/min. When the intermediate superheater pendants have been heated to a selected temperature level, a steam flow is passed through these intermediate superheater sections and also through the reheater. No steam, however, is passed through the finishing superheater section. Since steam is being taken from the steam drum, the firing rate is increased to maintain the same saturation temperature rise. Those low temperature sections which have reached the desired temperature level are protected from overheating by the steam flow. The finishing superheater section is thereby heated at a higher rate than it normally would be until such a time as it reaches a satisfactory temperature level. Only at this time is steam passed through the finishing superheater section into the hot piping where the critical fatigue problem exists.

FIG. 3 is a startup diagram as anticipated for a big new steam generator-turbine unit and is believed to be self-explanatory when viewed in conjunction with the invention as a whole. It shows in comparison with FIG. 2 very short available times for startup and defines the risks for very rapid temperature changes in the boiler if this is started up conventionally.

FIG. 4 is a schematic drawing which illustrates the significant features of the invention. The normal flow of steam is from the steam drum 2 to and through the low temperature superheater 3'. It then passes through the intermediate temperature superheater 4 and through the final superheater 5'.

The steam passes through the turbine through the final superheater outlet header 6, the steam line 7' and the turbine throttle valves 8'. A reheated steam section 9' is also shown.

With the unit bottled up and at a pressure between for instance 20 and atmospheres depending on pressure at and time since last operation, it is fired at a rate to increase the saturation temperature at a predetermined rate, for instance 3C./min. The metal temperature of the intermediate superheater outlet header 11' is measured. At the same time the metal temperature of for instance an outlet leg of the intermediate superheater section 4' is measured at a location in the gas stream. The temperature of this metal should normally be within a few degrees of the temperature of the steam inside the tube. The metal temperature is preferred to the steam temperature since the absence of any substantial steam flow at this time makes the measurement of the steam temperature of questionable value. Furthermore, evaporation of condensate in the loops could upset the temperature measurement on some occasions.

When these two measured temperatures are sufficiently close to avoid a thermal stress problem, or some other criteria chosen to protect the superheater in the intermediate superheater outlet header 11' are fullfilled, steam flow is started through certain portions of the unit. This is accomplished by opening the bypass valve 12 so that steam flows from the steam drum 2' through the superheater sections 3' and 4' to the cold reheat inlet header 13. The steam flows through the reheat sections 9', the reheat outlet header l4 and may be passed to the condenser through valve or passed to atmo sphere as desired. At this time no steam flows through the finishing superheater 5 nor through the steam line 7' and the turbines.

Due to the evaporation taking place in the steam drum, the firing rate must be increased to maintain the same rate of temperature rise in the waterwall circuits. This, in turn, increases the gas temperature entering the superheater sections. The intermediate superheater and reheater sections are protected from overheating by steam flow while at the same time the finishing superheater section 5' is heated more rapidly. The temperature of the the metal of the superheater outlet header 6' is measured as well as the temperature of an outlet leg of the finishing superheater section 5'. Steam flow is not admitted through the finishing superheater section until such a time as these two temperatures are sufficiently close to avoid thermal shock in the superheater outlet header 6.

This steam flow is established by opening the bypass valve 16 which causes steam to flow through the entire superheater section, the main steam line and the reheat lines. Presumably a desuperheater would be required in this bypass. At the same time as this bypass valve 16' is opened the bypass valve 12 which was earlier opened is now closed either at once or after some short period of time which will suit the steam generator as determined by tests or other criteria. Each of these bypass valves can be sized to provide the proper steam flow for the conditions which they are to satisfy. It should be noted that with a fixed opening of either one of these valves the steam flow through the valve increases as boiler pressure increases. Since the heat of vaporization decreases at high pressure, this increased evaporation is obtained with approximately the same firing rate so that the rate remains approximately constant over the range.

The valve 12 is controlled by a controller 31 responsive to the temperature of the intermediate superheater tubes and the respective outlet header as indicated by dashed lines in FIG. 4. The valve 16 is controlled by a controller 32 responsive to temperatures of the final superheater header and tubes as indicated by the dashed lin es.

It is, however, also so that the water volume in the boiler is approximately constant while the density as well as the heat of vaporization decreases at higher pressures. These effects can be approximately balanced against the increased flow at higher pressures so that the firing rate to maintain the present temperature increase can be kept at an almost constant level regardless of the pressure in the boiler in the range which is of interest.

The actual setting of anyone of the valves, especially the last bypassvalve 16, can however also be governed by for instance a steam flow measurement if this is considered as convenient.

The method described gives the fastest possible heating up of the superheater material and will that way as soon as possible in a safe way permit steam to be taken out from the unit without thermal shock problems in the thicker parts of the headers. it should be noted that even after the opening of the second bypass valve 18, it is still possible to increase the temperature of the saturated system at a preset value. This means that still a certain so-called overfiring or a higher firing rate than corresponding to the actual steam output can be maintained. This gives an increasing temperature not only in the saturated system but also in the superheater sections. The result is that the steam outlet temperature can be raised to a level where a safe and fast turbine loading can be obtained.

Preferably the superheater outlet header 6' should be insulated even though it is located in the dog house. This would permit the superheater outlet header metal temperature to remain approximately the same as that of the steam piping. By these means the thermal stresses may be avoided in all the high pressure sections at the same time. It should be noted that if no thermal stress problem exists in the intermediate superheater outlet header ll, that portion of the startup can be omitted with bypassing steam flow being based on some other criteria The significant portion of the invention being not only this basic concept which is applied to the intermediate superheater section but the use of this concept in the final section while bypassing steam around the final section and the turbine.

FIG. 5 represent gas and metal temperatures at various firing rates during startup. They illustrate the rate at which the superheater sections change temperature as the required heat is stored in the metal of the various sections.

ln the system may also a boiler throttle valve 20'a be included which could be used, if desired, to move the temperature drop due to throttling from the turbine throttle valves to this location upstream of the superheaters. If pressure downstream of the boiler throttle valve is to be less than about 400 p.s.i. at any time, it may be preferable to locate this valve 20b downstream of the first superheating section so as to avoid the formation of water-stream mixtures.

It should also be noted that the methods described very easily can be built up to a system which will allow almost any kind of boiler-turbine combination to be started up in the most convenient way, comprising the right criteria to allow an optimal operation of startup valves, shortest possible starting up time giving lowest startup heat losses etc.

Such a system could comprise-as can be seen by the description already made-the following features:

1. Criteria such as temperature measurements in the first sections after the furnace properly compared to header temperatures which will allow an initial startup valve to be opened as fast as possible and preferably to a preset value.

This means:

1.1 protection of those superheater sections which are most subjected to the risk of being overheated.

.2 possibility to increase the firing rate to values not permissible without a steam flow through these sections, giving a rapid heating of those sections not yet subjected to steam flow.

1.3 protection of the reheater as this part also will have a steam fiow. This part of the steam generator is otherwise subjected to very rapid and mostly uncontrollable temperature changes.

1.4 that if the burner ignition or the firing rate is not established quite as programmed for the startup sequence there will still be a protection for the first superheater sections after the furnace giving steam flow through these at the right moment.

2. Criteria such as the described method of metal temperature measurements of the final superheater section properly compared with the temperatures of the outlet header and/or of the steam piping, and possibly with some temperature in the turbine, will allow the second bypass valve 16' to open.

This means:

2.1 protection of the superheater when steam flow is admitted as soon as possible and not later than necessary.

2.2 protection of the headers and steam piping against thermal shocks.

2.3 together with the described method of operating with a certain degree of overfiring a controllable method to increase the steam temperature and also all other temperatures in the unit to a level which will allow a safe start and loading of the turbine.

3. Criteria such as the temperature of the turbine chest combined with the information from the boiler, can allow the rolling and the subsequent loading to be made at a time and in a way which will give the fastest possible subsequent turbine loading. The information can be handled as follows:

a. The actual temperatures and pressures of the boiler outlet parts including the superheaters and the turbine can give the criteria for the turbine to be rolled up and synchronized.

b. The first steam flow through the turbine is relatively small and the rolling can many times be made while the steam from the boiler still has a relatively low temperature, sometimes lower than the metal temperature in the turbine.

At the latest when the loading begins should, however, such a state have been reached that information from the boiler, such as availability of enough firing, a high enough steam temperature and temperature gradients in an allowed range, makes a sufficiently fast loading possible. This type of operation is by experience shown as best for the turbines.

3.1 The described startup system will allow a safe startup of a unit consisting of boiler and turbine combining the startup system for the boiler as described with a method to govern the turbine operation following information taken from both the boiler and the turbine.

3.2 The described startup system may be extended and combined with a so-called boiler-throttle valve 'a and 2071 respectively. The startup and loading of the turbine can thus always be made by steam, the conditions of which give a temperature distribution in the turbine rather close to that at high load. If closed when the unit is taken out of operation it also prevents condensation in the superheaters and the piping system.

The system described is also well suited for other types of boilers such as suband supercritical ones-through boilers. The only difference between these systems and that described is that these systems must have a minimum throughflow of about 30 percent in the furnace or initial evaporating and fluidheating system, obtained by means of circulating and/or feed pumps. The firing rate in the initial steps must therefore possibly be based on some other additional criteria than the temperature increase in this system only. However, the basic concept of the invention as described together with the startup operation will be the same.

I claim:

1. Method of starting up a steam generator with a series of superheater sections, each with its respective header, after a limited operational interruption, when thick walled elements, such as headers, are elevated in temperature compared to surfaces of said superheater sections, comprising, precluding normal steam flow through said superheater sections and headers, monitoring the temperature of each of said superheaters and its respective header, allowing normal steam flow through each of said superheaters, serially, as the temperature of each superheater rises to a predetermined value in relation to the temperature of its respective header.

2. The method of claim 1 wherein said generator also includes a plurality of reheaters, when diverted from its normal path steam flows through one of said reheaters. 

